Sources for Public Domain Music
When the Public Domain Information Project began in 1986, even the names of songs in the public domain were "secrets". Now there are at least half a dozen expensive lists, a couple of mid-priced ones, and the list on this web site. There is also the free list you can construct for yourself by searching the web and visiting the reference room at your local public library. The good lists give you not only titles, but for each title they add composer(s) and the copyright or publication date.
Catalogs are available offering copies of public domain music for sale. Genuine public domain arrangements certainly should include complete bibliographic information that tells you the origin of the music being offered. You should receive copies of the title page and the copyright notice to verify the seller's claims about the source of the song If the music is, indeed, taken from a legitimate public domain source, there should be no problem with supplying documentation. You must have this information to present if someone challenges you on the use of the piece or the arrangement. If it becomes necessary to defend your use of the music, you will need to show that this is, indeed, not only a piece of music that is legitimately in the public domain, but also that you worked from an arrangement that was in the public domain at the time you were doing the work. Having this information at hand from the beginning will usually prevent anyone from taking you to court over an infringement accusation, saving you thousands of dollars in legal fees. Any dealer selling music copied directly from public domain sources who refuses to supply complete documentation should be suspect. Public domain music without proof of PD status is virtually useless to you and not worth the purchase price.
Collections and Compendiums
When public domain music is collected into compendiums and sold as "royalty-free music", the compendium does not always carry individual copyright notices for all the pieces. When a "compendium of public domain music" has been widely distributed, the publisher has already tested the public domain status of the music by publishing it without being challenged. Furthermore, by identifying it as royalty-free, he has intrinsically stated that once you purchase your copy of his collection, he expects no further income from you. You have bought all copyrights to the individual pieces in the collection including performance, arrangement, or recording. The only limit is that you may not republish his book. But make certain the compendium is produced by a reliable publisher for you are trusting the publisher's research rather than your own. You have no legitimate source for proof of public domain in your possession. If the publisher included a work still under copyright in error, you still will be liable for royalties. The ideal compendium would include the correct copyright information for each piece included.
Avoid compendiums or "fake books" printed by companies that put their own copyright on every piece in the book as well as on the collection. One of these contained several hundred public domain works in which the editor had "significantly" changed two measures of the melody line in every song. This created recognizable "arrangements" which he considered billable derivative works. Not only was the book absolutely useless for public domain research, the publisher might claim and collect royalties if his melody line were used. At this time we are not aware of a reasonably priced, adequately footnoted, royalty-free Public Domain Fake Book that would be genuinely useful to arrangers.